The use of hydrodynamic retarders as a means for applying braking torque to a vehicle transmission is well known. Typically, such retarders employ a rotary member connected to the hub of a transmission output shaft which is operative to circulate fluid between vane members on a stationary housing. The fluid circulation results in power absorption and retardation of the output shaft of the transmission. A reduction in vehicle speed typically results from such action, assuming that the roadway or other supportive surface of the vehicle demonstrates a frictional coefficient sufficient to support such braking torque.
It is generally known to employ a solenoid valve in hydrodynamic retarders to function as a pressure control valve under pulse width modulation as controlled by an appropriate transmission control unit such as a dedicated microprocessor or the like. The duty cycle at which the solenoid valve is actuated regulates the retarder capacity or the load applied to the output shaft of the transmission and, accordingly, controls the braking activity achieved by the retarder. Retarders of the known type and their associated control systems are shown, by way of example, in U.S. Pat. Nos. 4,768,401, 4,836,341, 4,711,328, 4,257,504, and 4,235,320. The mechanical structure of the previously known retarders are typically adaptable for implementation in accordance with the instant invention, but for a modification to the control valve spool which will be discussed below.
It has generally been known that the prior art retarders have operated in an open loop mode, not employing real time adjustments for feed back control to compensate for system variations to maximize retarder efficiency. The prior art retarders have not been adaptive to accommodate the effects of aging on system operations, responsiveness, and repeatability. Additionally, the previously known retarders have not been responsive to excessive vehicle deceleration rates to modify the retarding activity, nor have they been responsive to excessive heat buildup in the retarder/transmission to modify, alter, or suspend the retarding activity. Prior art retarders have also been known to aggravate transmission downshifting by adding excessive braking torque during the shifting operation, the same being inconsistent with smooth transmission operation during downshifting as required for passenger comfort.
Additionally, the previously known retarders have generally interfered with the transmission operation during a downshift when the retarder is concurrently seeking to achieve increased activity. In such situations, where both the transmission and the retarder are demanding increased volumes of transmission oil, any insufficiency in oil to satisfy both needs has resulted in ineffective and inefficient operation of both the transmission and the retarder. Previous attempts to circumvent such a problem have typically required the implementation of oversized oil pumps and reservoirs to assure sufficient oil volume to satisfy the needs of both, the same adding to the complexity and cost of the transmission system.
There is a need in the art for an electronically controlled hydrodynamic retarder for use with a vehicle transmission which overcomes the shortcomings of the prior art noted above.